An important problem in the biology of mammalian cell growth concerns the regulation of messenger RNA function. As a model for mammalian cell growth regulation we have studied the serum induced proliferation of contact inhibited fibroblast, Balb C-3T3 cells. Since mRNA molecules are associated with proteins (mRNP) through most of their life history in the cell, we sought to examine the regulation of mRNA metabolism at the ribonucleoprotein level of organization. During the transition from resting to proliferation state, there is a five fold increase in the rate of protein synthesis as measured in intact cells and in the cell free lysates. The poly(A) containing RNA molecules are markedly stabilized in serum stimulated cells. The transition from resting to growing state also shows characteristics changes in the size and cytoplasmic distribution of poly(A) segments of mRNA as well as the types of proteins associated with mRNP particles. Reduced rate of protein synthesis in contact inhibited cells appears to be largely due to an inhibition at the translational initiation step. Proposed studies are aimed at examining further the specificity of mRNP proteins and their role in the regulation of mRNA function and stability. Controlled S1 nuclease digestion of mRNP complexes followed by selection of the 3'-poly(A) fragments on oligo d(T)-cellulose will be used to determine the location of the main mRNA binding proteins. Modulations in the properties of mRNP proteins with altered cell growth conditions, will be determined on the basis of their labeling properties and migration in two-dimension gels. The abundance and translational efficiency of poly(A) containing and poly(A) lacking mRNA at the two growth conditions will be compared in cell free lysates. Possible role of mRNP proteins in translational regulation will also be compared in cell free lysates. These studies therefore will help explain the mechanism of translational regulation at different growth conditions in 3T3 cells and may be useful in understanding mammalian cell growth regulation in general.